During artillery fire, a forward observer is placed between the artillery pieces and a fixed target, typically at approximately 1 or 2 km from the target, the artillery pieces being placed to the rear at a distance typically between 5 and 50 km for terrestrial artillery fire. This observer, who has a direct view of the target, is initially responsible for determining the position of this target. This is referred to as extraction of coordinates of the target.
The latter are obtained in the following manner by means of a fixed but orientable optronic observation system such as a multifunctional scope or binoculars fixed to a tripod.
This observation system described with reference to FIG. 4 conventionally includes an observation channel that includes an image sensor 2 and a laser rangefinder channel referred to as the laser channel that includes a rangefinder 4; it also includes a display screen 1 common to the two channels on which appears the image 10 from the image sensor and on which is positioned a rangefinding crosshair R1 showing the aiming axis of the laser rangefinder, as can be seen in FIG. 1. This crosshair generally takes the form of a cross. The laser beam of the rangefinder is emitted in a very narrow sector typically of approximately 1 mrad, which imposes very accurate pointing of the laser. A harmonization, that is to say alignment of the axes of the laser channel and the observation channel, is factory-set; as a result of this the crosshair R1 is located substantially at the center of the screen 1. To measure the range of the target, the operator orients the binoculars so as to position the laser crosshair R1 over the image of the target after which rangefinding is effected by means of a user interface 7, for example by action on a pushbutton. The observation system is furthermore equipped with means 5 for measuring the orientation of the aiming axis (of the observation channel or of the rangefinding channel since they have the same axis), such as a magnetic compass, a goniometer or a gyrocompass, etc., or any other means, and positioning means 6 such as a two-antenna GPS system, for example.
This observation system is for example mounted on a tripod and therefore has a fixed geographical position and can be oriented. As indicated, the observer orients the observation system so as to make the crosshair R1 coincide on the display screen 1 with the image 10 of the target as shown in FIG. 1. They then operate the rangefinder to measure the distance D between the system and the target, at the same time as the measured position and orientation of the system are respectively calculated by the positioning and orientation means. The coordinates of the target are extracted from these three measurements and transmitted to the artillery pieces, for example by voice transmission.
A certain number of factors may lead to the first impact not being on the target:                Inaccuracy of the extraction of coordinates,        Defective adjustment of the artillery piece,        Temperature of the powder and the barrel,        Wind,        Etc.        
If the first impact is not on the target, the second mission of the forward observer is to provide the operators of the artillery pieces with the parameters necessary for the determination of artillery fire corrections to be made to achieve a second strike, this time on the target. The forward observer provides three parameters, as indicated in FIG. 2:                target observation bearing,        distance to right or left between impact and target, which is 10 m more to the left in the example in the figure,        distance of the impact in front of or behind the target, which is 15 m closer in the example in the figure.        
These parameters are calculated on the basis of the measurement of the following elements, knowing that the measurement of the distance D resulting from the coordinate extraction step is considered sufficiently accurate:                Distance D′ between the observer and the impact,        Orientation offset between the impact and the target: in practice this is an offset in bearing.        
To prevent the image of this second strike being outside the target and therefore to minimize collateral damage, the calculation of these parameters must be as accurate as possible, notably with an angular accuracy of one mrad, the accuracy in respect of D′ being sufficient.
In fact, the distance D′ is obtained by laser rangefinding with sufficient accuracy of the order of ±5 m.
At present there are two devices for determining the offset in bearing between the target and the impact:                The magnetic compass,        The goniometer.        
The magnetic compass is a device sensitive to the terrestrial magnetic field and enables magnetic North to be determined at a location; it is then easy to deduce geographical North at this location, by adding the magnetic declination. With the aid of a magnetic compass it is possible to measure by pointing at a target the observation bearing to that target. By pointing toward the impact and effecting a subtraction, it is possible to determine the offset in bearing between the target and the impact. The advantage of the device lies in its compactness and its lightness. It is easy to integrate into more complex systems such as multifunction binoculars, for example. Its disadvantage is linked to the sensitivity of this type of sensor, which is extremely sensitive to interference and in the best possible scenario is unable to guarantee a measurement to within less than 10 mrad. Now this accuracy of 10 mrad is highly insufficient since the order of magnitude of the artillery fire corrections that it is required to provide is 1 milliradian.
The goniometer is a relative angle measuring device. It makes it possible to measure a relative angle with great accuracy, less than one mrad. By successively pointing the sight line of binoculars at the target and then at the impact point, it enables the offset in bearing to be measured with the required accuracy. The disadvantage of the goniometer is that it is heavy and bulky, which is a penalty for tactical hardware, and that it adds a non-negligible cost to the system.
It is also possible to calculate these correction parameters using an observation system such as binoculars or a scope, the display screen of which is provided with a micrometric crosshair R1, i.e. one completed by small markers, the distance between two markers defining a field of view, as represented in FIG. 1. Observers evaluate the offset in bearing and in elevation for themselves as a function of the offset that they observe on their display screen 1 between the micrometric crosshair R1 positioned over the image 10 of the target and the image 11 of the impact on their screen; however, this evaluation by the observers themselves cannot achieve the required accuracy of the order of 1 mrd. By rangefinding, they then measure the distance after first orienting their observation system toward the impact, that is to say positioning the crosshair over the image of the impact.
Consequently, there remains at present a requirement for a system simultaneously satisfying all the aforementioned requirements, in terms of the accuracy of the corrections to be made, compactness, lightness and cost.